It is known in the art to provide fluid working machines in which the flow of working fluid into and out of working chambers of cyclically varying volume (e.g. piston cylinder arrangement) is selected on each cycle of working chamber volume by actively controlling the opening or closing of at least one electronically controlled valve, to select the net displacement of working fluid by the working chamber on each cycle of working chamber volume.
This is known, for example, from EP 0361927 (Salter and Rampen) in which a low pressure valve (LPV) which regulates the flow of working fluid between a working chamber and a low pressure manifold is actively controlled to enable a pump to carry out either an active cycle or an inactive cycle. EP 0494236 (Salter and Rampen) developed this concept and introduced an actively controlled high pressure valve (HPV) which regulates the flow of working fluid between a working chamber and a high pressure manifold, enabling a motor to carry out either an active cycle or an inactive cycle and also enabling a fluid working machine to carry out either pumping or motoring cycles.
By active cycles we refer to cycles of working chamber volume which lead to a net displacement of working fluid from the low pressure manifold to the high pressure manifold, or vice versa. By inactive cycles, we refer to cycles of working chamber volume in which there is no net displacement of working fluid between the low and high pressure manifolds. Inactive cycles such as those described in EP 0361927 and EP 0494236 involve a working chamber receiving working fluid from the low pressure manifold and venting the same amount of working fluid back to the low pressure manifold, so that there is no net displacement of working fluid, although it is also known (e.g. from WO 2007/088380, Stein and Caldwell) to carry out an inactive cycle by keeping a working chamber sealed throughout a cycle of working chamber volume, from one minimum of working chamber volume until the next, so that there is no flow to or from any manifold.
In machines of this type, the LPV is actively controlled to select between active and inactive cycles, and in some embodiments to control the fraction of maximum stroke volume which is displaced during active cycles. In order to enable active control, each LPV is electronically controlled and has an, actuator, typically a solenoid, which is coupled to a valve member. A solenoid may for example act on an armature which is coupled to the valve member (without necessarily being rigidly connected) through a valve stem. The HPV is also typically actively controlled in which case it also has an electronically controlled actuator, typically a solenoid, which is coupled to a valve member. However, in the case of a pump, the HPV can be operated in a solely passive way, for example, it may be a normally closed, pressure openable check valve.
By active control we include the possibility of a valve being actively opened, actively closed, actively held open or actively held closed. A valve may be biased open (normally open) or closed (normally closed). An actively controlled valve may also move passively in some circumstances. For example, a LPV may be actively closed but open passively when the pressure in a cylinder drops below the pressure in the low pressure manifold.
In order to control the net displacement of a working chamber, one or more actuation signals are sent to the valve actuators of the respective LPV (and for motoring also the HPV). The actuation signals are selected so that the resulting displacement of the working chambers closely follows a target. The target displacement may change rapidly and as displacement decisions are made frequently, the actual displacement of the machine can vary very rapidly. Hence this type of machine can rapidly vary displacement and shaft torque, while operating in an energy efficient way. Examples of algorithms which can be employed to select active and inactive cycles to meet a target demand in this type of machine are, for example, disclosed in WO 2015/040360 (Caldwell et al.) and WO 2011/104549 (Rampen and Laird).
The efficiency of these machines requires careful control of the timing of valve opening and closing and so the function of the machines can be degraded if valve timing is not reliable. However, with this type of machine, we have identified that problems can arise if a valve is not used for a sufficient period of time. A valve which has not moved for a sufficient period of time may fail to move when it is commanded to do so, or may respond more slowly, or less predictably, than when the valve is being regularly used.
We have found that delays and irregularities in valve member movement can arise from a number of factors. Without wanting to be bound by theory, problems arising from a period of inactivity include:
1. A variation with time in the amount of damping arising from a squeeze film at the axial interface(s) of a valve member (particularly the LPV). A primary interface is the broadly axial face presented by the sealing line of the LPV on its corresponding valve seat. During a period of inactivity, the LPV valve member is thought to drift from a lightly seated position towards a more firmly seated position, pushing out oil from between the valve member and valve seat, thereby decreasing the thickness of the oil film leading to an increase in damping. In turn, this provides a greater resistance to unseating of the valve member.
2. The thickness of the oil film around the valve member also affects the transition from boundary to dynamic fluid lubrication to hydrodynamic lubrication. When there is a prolonged period of no valve movement, boundary lubrication acts between the LPV and primarily the bore, and may involve metal to metal contact. As the valve member starts to move, working fluid is drawn into the intermediate space, a dynamic fluid wedge/hydrodynamic filed is created and the lubrication mode switches instead to dynamic fluid/hydrodynamic. The damping forces are in part influenced by this transition which is itself influenced by the thickness of the oil film.
3. Inactivity may also lead to concentricity drift, possibly arising due to tilting of the low pressure valve. The longer that the LPV remains stationary, the more likely it is for the valve member or other parts connected to the valve member (e.g. armature or valve stem) to suffer from movement to one side of the bore within which they travel, leading to a thicker oil film on one side and thinner oil film on the other. The variation in film thickness will likely increase the overall damping force on the travelling member.
4. Another factor may be stick-slip, which is the relaxation-oscillation phenomena involving the build-up of elastic energy as a tangential force is applied. The tangential force increases until it exceeds the interfacial resisting force, at which point instant slip occurs. The LPV suffers primarily a single stick event per stroke, and the stick-slip process repeats only when LPV returns to a seated position and is again demanded to open.
5. If a solenoid valve actuator is used less frequently, it will be cooler than if it is regularly used, meaning that it will have a lower electrical resistance therefore higher efficiency and greater current when actuated.
6. Magnetic materials may be influenced by remanence or remanent magnetization. The magnetic flux circuit established when a solenoid is energized may remain magnetized after the current is removed.
7. After actuation of a solenoid, eddy currents may remain circulating for a period of time. Eddy currents will therefore persist between frequent actuations of a solenoid controlled valve but will die away after a period of inactivity. After a greater period of inactivity, the energy required to fully re-establish the magnetic circuit in a solenoid increases.
Many of the above effects which have a detrimental effect on valve performance are closely time related, and thus their influence typically peaks at machine start up (when the machine has been dormant for long period of time).
Hence, when a valve actuation signal is sent to the actuator of a valve which has been inactive for a sufficient period of time, the amount of fluid displaced by the respective working chamber may be more or less than required, or the required tolerance in valve response time may cause the valve timings to have to be set suboptimally to avoid a risk of failure.
Accordingly, the invention seeks to address problems arising from unreliable valve actuation arising from periods of inactivity of a valve. This will enable the resulting machine to be more stable, more reliable, more efficient and/or more tolerant of variations in environmental factors (e.g. temperature).
Note that although a given valve will have been inactive before start up of the machine it is in the nature of these machines, which make decisions as to displacement on cycle by cycle basis, that individual working chambers and their valves may remain inactive for a period of time even if the machine is operating continuously with non-zero displacement being met by other working chambers carrying out active cycles.